Electric power distribution networks are used by the electric utilities to deliver electricity from generating plants to customers. Although the actual distribution voltages will vary from utility to utility, in a typical network, 3-phase power at high voltage (345,000 volts phase-to-phase) is delivered to multiple transmission substations at which transformers step this high voltage down to a lower 3-phase voltage (69,000 volts phase-to-phase). This 69,000-volt 3-phase power then feeds multiple distribution substations whose transformers further step down the voltage to the distribution voltage (12,470 volts phase-to-phase) and separate the power into 3 single-phase feeder cables. Typically, these 3 feeder cables operate at 7,200 volts phase-to-ground and are designated as phase attributes A, B, and C. Each of these 3 feeder cables branch into multiple circuits to power a plurality of local pole-mounted or pad-mounted transformers which step the voltage down to a final voltage of 120/240 volts for delivery to commercial and residential customers.
In many cases, the final 7,200-volt distribution network utilizes underground (i.e., buried) cables. These cables are typically known as Underground Residential Distribution (URD) cables. Typical URD cables are shown in FIG. 1.
In a typical URD cable 20, a center conductor 22 is surrounded by an inner semiconductor sheath 24. Inner semiconductor sheath 24 serves to relieve electrical stress by spreading out and making the electrical field more uniform.
Inner semiconductor sheath 24 is surrounded by an insulator 26. Insulator 26 has significant high-voltage insulating properties to minimize the overall size of URD cable 20. Typically, insulator 26 is formed of a polymeric material, such as polyethylene.
Surrounding insulator 26 is an outer semiconductor sheath 28. Like inner sheath 24, outer sheath 28 serves to relieve electrical stress by making the electrical field more uniform. Making the electrical field more uniform protects insulator 26, which would otherwise be more likely to break down.
Outer semiconductor sheath 28 is surrounded by a shield formed of a plurality of neutral conductors 30. Neutral conductors 30 together serve as a return line (ground wire) for center conductor 22. Neutral conductors 30 are surrounded by and embedded within an insulating jacket 32. However, many older URD cables are not insulated using insulating jacket 32.
URD cables 20 are terminated using load break elbows 100 illustrated in FIG. 2. Elbow 100 is composed of insulated material 105 with pulling eye 110. A short length of insulating jacket 32 of URD cable 20 is removed, neutral conductors 30 are folded back and twisted together to form grounding wire 125 which is connected to earth ground. A compression connector 115 is attached to bare center conductor 22, and inserted into insulated material 105. Male contact pin 120 is screwed into compression connection 115.
Most modern elbows also incorporate a capacitive test point 130 covered by removable cap 135. When cap 135 is removed, test point 130 capacitive couples to center conductor 22 which allows sensing the voltage of center conductor 22.
A mating insulated elbow bushing (not shown) is mounted inside a cabinet. Using an insulated hot stick, a lineman grips the pulling eye 110 to insert or remove elbow 100 from the cabinet elbow bushing, thus making or breaking the URD cable circuit.
A simplified small portion of a typical URD circuit is illustrated in FIG. 3. Upstream single-phase attribute feeders A, B, and C branch out using junction cabinet J1. That is, phase A URD cable elbow E1 is pressed onto bushing B1 which is permanently mounted in cabinet J1. Bushings B2 and B3 are both also permanently mounted in cabinet J1 and are connected to B1. Thus, cabinet J1 expands single phase A URD cable into 2 phase A URD cables. Likewise, phase B and C URD cables are also expanded.
Also illustrated in FIG. 3 is downstream padmount transformer cabinet T1 which contains permanently mounted bushings B4 and B5 which are connected together and to the primary (high voltage) input of a step-down transformer. The secondary (low voltage) output supplies the final 120/240 volt power to the residential customer. A length of URD cable carries power from upstream junction cabinet J1 to downstream padmount cabinet T1 using elbow E3 connected to bushing B3 in junction cabinet J1 and elbow E4 connected to bushing B4 in padmount cabinet T1. Another length of URD cable typically carries power to a second downstream padmount cabinet (the next one in a long chain of downstream padmount cabinets) using elbow E5 connected to B5 in padmount cabinet T1.
Utilities assign and tag each length of URD cable with a unique number near the elbow to identify which elbows are connected to each section of URD cable. This is required so that a lineman can disconnect the correct elbow if a portion of the URD circuit must be de-energized. For example, assume the chain of downstream padmount cabinets after T1 need to be disconnected by pulling elbow E5 off of bushing B5. If the padmount tags were missing or incorrectly indentified, the lineman might pull elbow E4 which would disconnect all customers connected to T1.
Unfortunately over time, equipment failures and new construction can lead to tagging errors. Utilities would like to re-confirm URD tagging accuracy, but there is currently no way to accomplish this short of pulling elbows to determine which elbows belong to a length of URD cable. Since the URD cables are buried, they cannot be visually seen where they originate. Although cable locator equipment can trace URD cables from one cabinet to another, this equipment cannot identify which elbow connects to the cable being tracked since both the input and output elbows are connected together via the two cabinet bushings. For example in cabinet T1, bushings B4 and B5 connect elbows E4 and E5 together.
Junction cabinets are implemented each time a URD cable branch is required. Long chains of padmount transformer cabinets snake through residential neighborhoods, each serving one or a few houses. Therefore, URD cable circuits are extensive. Junction cabinets sometimes contain multiple single phase feeders and some of the feeders could split into multiple branches going to additional junction cabinets and to multiple different chains of padmount transformer cabinets.
Currently no method or apparatus exists to quickly and easily identify elbow connectivity on live URD cable circuits so as to accurately confirm if each elbow tag is either correct or needs to be re-tagged. Even small and medium size utilities have thousands or tens of thousands of URD elbows in active service. Conducting an utility wide elbow connectivity survey today without better tools and techniques (as disclosed in this patent application) would be prohibitively expensive and is rarely (if ever) performed